1. Field of the Invention
The present invention relates to a rotation information detecting apparatus and method which measure the rotation information of a rotating object and are preferably applied to, e.g., a rotary encoder for measuring the angular displacement (rotational displacement) of a rotating object.
2. Related Background Art
In a conventional rotary encoder for detecting the rotation information of a rotating object, a disk-like or cylindrical scale is used, and light or magnetism is magnetically, optically, or capacitively detected through the scale and a rotational angle information pattern arranged to oppose the scale, thereby detecting the rotation information on the basis of the detection signals. In this rotary encoder (first rotary encoder), a member (scale) obtained by forming a pattern associated with rotation information on a disk substrate in advance is mounted on a rotating object, and the rotation information of the rotating object is measured by using the scale.
In a magnetic encoder using a magnetic scale and a magnetic detecting means, a magnetic medium of the write once and read many or rewritable type is used as a scale.
An optical rotary encoder (second rotary encoder) is proposed in Japanese Laid-Open Patent Application No. 4-70509. According to this official gazette, an optical disk of the write once and read many or rewritable type is used as a scale. This scale is mounted on a rotating object with no position information (format pattern) for detecting an angular displacement amount being recorded on the scale. A pattern associated with rotation information is written on the optical disk.
In order to obtain a high-precision rotary encoder, the following two conditions must be met.
(1) A high-precision pattern (radial pattern) must be formed on a scale. That is, no pitch error in the circumferential direction is allowed, and the pattern formed on a circumference must not have any eccentricity with respect to the rotation center. PA0 (2) A pattern on the scale and its detecting means must be positioned in correspondence with the rotation of a target with a high precision.
In general, in the first rotary encoder, a high-precision scale is required, and high-precision parts are required to manufacture the scale. For this reason, as the constituent elements of the scale, high-cost parts are used. In addition, in mounting the scale and detecting means, high-precision alignment is required.
In the second conventional rotary encoder, a scale having no pattern is mounted, and the scale is rotated to form a pattern. In this method, there is no chance of misalignment between the geometrical center of the pattern and the rotation center of the target. However, in order to meet condition (1), a proper apparatus and means must be selected and used to form a pattern.